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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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THE EARTHQUAKE ADVANCE THEORY         187
seem to us to have such direct bearing on the problem of glacier motion, and to point to viscous flowage as the mode of motion, that we deem them worthy of specific statement in this connection.
Evidence of Viscosity. If it is granted that there has been actual transfer of material in the advancing glaciers of Yakutat Bay, and the facts render no other conclusion possible, we may ask whether any of the other theories of glacial motion than that of viscous flowage are capable of accounting for the abrupt transfer of ice to such a •degree and extent as is observed? As a result of our consideration of this question we have been forced to the conclusion that no other mode of motion than viscous flowage can be appealed to in explanation of the thickening, spreading, and advance of these glaciers. We are, of course, open to conviction of error in this conclusion if it can be shown by those who accept other theories of glacial motion that any one of them can explain the phenomena of the advancing glaciers. The motion, whatever its nature may have been, must explain the faulting and linear crevasses on the Lucia Glacier, the increase of breaking along the margin and the piling up of ice on the stoss side of the Lucia nunatak. It must explain the system of crescentic crevasses in the Atrevida Glacier piedmont bulb, and the pronounced thickening and spread of this bulb and that of the Variegated Glacier. Further, it must account for the rapidity of all the changes and the notable forward motion of the fronts of Haenke and Hidden Glaciers, one and two miles respectively. All these phenomena are but natural and simple results of viscous flowage if once the possibility of this is granted; are they also capable of explanation on the basis of other theories of glacier motion? We cannot conceive that they are and, consequently, because of the failure of other hypotheses are driven to believe in viscous flowage as the mode of motion in these advancing glaciers.
In addition to this consideration, there are some significant facts bearing on the problem of the mode of motion of the glaciers. One of these is the appearance of clear ice areas in parts of the glacier surface which prior to the advance were overspread with ablation moraine. This is observed on the Variegated Glacier bulb where previously-existing moraines were swept forward, the area of clear ice extended beyond the mountain valley to a portion of the surface formerly covered by moraine, and a new area of clear ice introduced between moraines far out in the area of former ablation moraines. It is even more clearly illustrated in Atrevida Glacier, where an extensive area of clear ice appeared in 1906 in the midst of moraine-covered ice and in a position where in 1905 there was a uniform sheet of ablation moraine. Here also the ice layers are highly inclined as if there was uprising from below. We canTeadily explain these facts on the hypothesis of viscous under-flowage and local uprising of ice from below, pushing the moraine-covered ice aside in certain favorable localities, the ice becoming crevassed after it rose to the surface and was released from strain, as 1906 photographs show. We are wholly at a loss to account for it on any other mode of ice motion.
That the lower ice of a glacier is in a state resembling plasticity, and wholly in contrast to the upper ice, is suggested by the crevasses which break so many glacier surfaces. But in the advancing glaciers of Yakutat Bay we have a better chance of studying and understanding these crevasses than usual. They did not exist before the advance, they were then developed in extraordinary numbers, and then at once their cause ceased and they began to be destroyed by the ablation of the ice surface. The conditions under which these crevasses were developed, we may fairly assume, were